No one expects Haley to drop out of sight after January. The only question is when she will return to the political fray. The timing of her announcement would indicate that this could take place sooner rather than later. While it is certainly the case that aspiring candidates for president begin to build their campaigns and policy teams shortly before or after congressional midterm elections, and Haley clearly will not do so, this need not necessarily be the case. Haley could discreetly build a staff once she has left office at the end of the year, and then hold off announcing her intentions until mid-2019. George W. Bush withheld making any official announcement of his intention to run until more than seven months after the congressionals; Haley could do the same.
Haley certainly can be expected to run if Trump does not; even if he does, she could mount a powerful insurgent campaign against him. She is a true conservative with views ranging from abortion to Israel to a strong defense that are shared by the Republican base. And her stand on immigration is sufficiently moderate so as to avoid alienating that base. Should she make a run for the presidency, that in and of itself might convince Trump to call it quits, much as President Lyndon Johnson did in 1968. And then America might find that it has elected its first woman president.